1. Field of the Invention
The present invention relates to a light beam scanning device in which a f-.theta. lens system is disposed such that a deflective reflecting surface and a surface to be scanned are set in an optical conjugate relationship, and a linear convergent light beam which is deflected at a constant angular speed by the deflective reflecting surface is transmitted through the f-.theta. lens system to be converted at a constant speed and then is focused on the scanning surface.
2. Description of the Prior Art
A light beam scanning device to which the present invention is applied is arranged as shown in FIG. 1A. That is, a laser beam or another light beam 1 which is modulated in correspondence to input information is transmitted through a collimating lens system or another lens system 2 and then incident as a parallel linear convergent light beam on a deflective reflecting surface 3. The light beam 1 is deflectively reflected at a constant angular speed by the deflective reflecting surface 3 and then incident on an image-forming lens system 4 (to be referred to as an f-.theta. lens system hereinafter) having f-.theta. characteristics to be converted into constant-speed motion. Thereafter, the resultant light beam 1 is focused on a surface to be scanned 5 of, e.g., a photosensitive drum located at a focal point of the light beam 1 and scans the scanning surface 5, thereby forming an electrostatic latent image corresponding to image information on the scanning surface 5. (Note that the shapes of two lenses 41 and 42 which constitute the image-forming lens system 4 are portions according to the present invention and hence are not the prior art.)
A device of this type is already known. In such a device, a rotatory polygonal reflecting mirror 30 which rotates at a constant speed about a rotating shaft 3A supported in a direction perpendicular to a deflection plane is generally used as the deflecting means. However, according to such a deflecting means, inclination tends to occur in a direction perpendicular to the deflection plane by small errors between the vertical axis of the reflecting mirror 30 and the rotating shaft 3A and between the rotating shaft 3A and the deflective reflecting surface 3, and this inclination appears as beam pitch variations on the scanning surface 5.
The above pitch variations can be eliminated by using the f-.theta. lens system 4 in which the deflective reflecting surface 3 and the scanning surface 5 are set in the optical conjugate relationship.
Accordingly, in such f-.theta. lens system 4, a deflection plane (to be referred to as an X plane hereinafter) by which the light beam 1 is deflected and a sectional plane (to be referred to as a Y plane hereinafter) perpendicular to the deflection plane must have different optical characteristics, and scanning surface at a constant speed of the beam must be realized while preventing the pitch variations and image surface curvature of the beam on the scanning surface 5. Since it is difficult to constitute such a lens system by a single lens, a plurality of lenses are normally used.
For example, the following technique is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 50-93720. That is, in this technique (to be referred to as a first prior art hereinafter), a lens having distortion characteristics for realizing conversion at a constant speed is disposed at the side of the deflective reflecting surface 3, and in order to eliminate the pitch variations in the light beam to be focused on the scanning surface 5 to scan the surface, a cylindrical lens having refractive power in the Y-plane direction is disposed at the side of the scanning surface, hereby realizing conversion at a constant speed of the beam and preventing the pitch variations. However, when such an arrangement is adopted, the beam subjected to conversion at a constant speed is transmitted through the cylindrical lens again. Therefore, as the cylindrical lens is disposed closer to the deflective reflecting surface 3, the image surface curvature tends to occur more often.
Accordingly, in the above arrangement, a good image with less image surface curvature cannot be obtained unless the cylindrical lens is disposed closer to the scanning surface 5.
However, in order to dispose the cylindrical lens closer to the scanning surface 5, the cylindrical lens must be elongated in a deflecting direction in correspondence to a scanning width. As a result, it becomes difficult to achieve high manufacturing and assembly accuracies, and the f-.theta. lens system is undesirably enlarged.
In order to eliminate the above drawbacks, for example, the following technique is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 56-36622. That is, as shown in FIG. 5, according to this technique (to be referred to as a second prior art hereinafter), a spherical lens 43 and a toric lens 44 are disposed sequentially from the deflective reflecting surface 3 toward the scanning surface 5, and the f-.theta. lens system 4 is constituted by the two lenses. Note that the toric lens 44 is a lens having a positive or negative power in each of X- and Y-plane directions in a surface perpendicular to an optical axis of the lens and having different powers in the X- and Y-plane directions.
In order to minimize the side of (flatten) such a device, a lens is preferably formed flat to reduce a lens width (height) in the Y-plane direction. However, if the spherical lens 43 is to be used, it is very difficult to polish a lens material which is cut out flat. Therefore, as shown in FIG. 6, a polished spherical lens 43A is cut parallel to its optical axis so as to obtain a rectangular portion, and only this rectangular central portion 43 is used.
However, since only part of the polished spherical lens 43A is used, a manufacturing cost is naturally increased, making it difficult to realize cost decreases. In addition, it is difficult to cut the spherical lens 43a parallel to the optical axis, and a manufacturing error is often generated.
Since both lenses 43 and 44 have the refractive power in both the X- and Y-plane directions, they must be disposed at predetermined positions so that their optical axes coincide with each other in both the X- and Y-plane directions. As a result, assembly and adjustment become troublesome and may cause an assembly error to occur.